The invention concerns a device used for mounting and alignment of a Christmas tree which can be lowered from a platform to a wellhead which is located on a seabed. The Christmas tree has a funnel which is widened downwardly, orientation bodies for angular positioning of the Christmas tree relative to the wellhead, and connection bodies for connecting the Christmas tree to the wellhead. The device comprises a side element of a drilling template, which has a guiding device which is arranged to receive a guide pipe, the guiding device comprises a conical funnel, and the guide pipe is arranged to receive the wellhead and comprises a cylindrical adaptor.
From the prior art it is known that when mounting a Christmas tree on a wellhead, guidelines are employed which extend from the platform to the drilling template, the guidelines being connected at the bottom to guide posts on to which sleeves of the Christmas tree are passed, and which provide a precise alignment of the Christmas tree in a horizontal plane relative to the drilling template.
The use of guide lines and a drilling template is described for instance in GB 2 285 274 which discloses a drilling template comprising a central part and arms that may be swung outwardly. Each of the arms is arranged above a well and a manifold is mounted at the central part. The well and the manifold are interconnected through pipes.
Each well is completed by firstly introducing a drill string through an arm. and boring a hole into the formation underneath the arm. Thereafter a guide pipe or conductor is introduced into the hole and cemented there whereby it is able to carry heavy equipment. The top of the guide pipe is welded to a guide pipe housing which comprises outer orientation elements such as pegs, which are adapted to be introduced into corresponding slots in the arm. A guide base or plate is fixed to the guide pipe housing. During the lowering of the guide pipe/plate assembly through the opening in the arm the orientation elements are providing a rotation of the assembly until its position is correct with respect to the central part of the drilling template.
Guide posts wherefrom guide lines are running to a platform at the sea surface are provided at the base or plate.
The guide pipe housing is provided with internal steps in such a way that it may support casings which are introduced into the well.
When a Christmas tree is to be mounted at the well, it is lowered and guided by means of the guide lines in such a way that the Christmas tree is oriented correctly with respect to the manifold at the central part of the drilling template. This is very important because the Christmas tree and the manifold have pipe connector parts which shall be interconnected during the lowering of the Christmas tree, and the tolerance with respect to the mutual angular orientation of the Christmas tree and the manifold is small.
The use of guide lines for drilling templates at very large depths is difficult because the guide lines become tangled up. It is also difficult to keep the guide lines tensioned sufficiently.
In the case of drilling templates which are employed at very great depths where no guidelines extend therefrom to the platform, there are difficulties involved in such an alignment.
The object of the invention is to provide a mounting device of the type mentioned in the introduction which enables the above-mentioned alignment to be implemented.
The characteristics of the mounting device according to the invention are presented in the characteristic features indicated in the claims.